Crystallization of compounds from solution is a separation and purification method used in the chemical, food, and pharmaceutical industries, particularly for the production of active compounds or their intermediates. Some of the goals of the crystallization process include producing a product meeting a desired purity level and also a product having the desired crystal size and size distribution. Crystallization from solution may be conducted as a batch process or as a continuous process. Batch crystallization equipment and operation is fairly simple but requires a significant investment of both time and money between batches. Additionally, batch crystallization suffers from quality control issues due to the lack of a steady state during the batch crystallization process. Continuous crystallization may be used for large volume commodity type materials with loose tolerances. This is because continuous crystallization does not typically permit adequate quality control to yield crystals suitable for use in industries demanding a high level of crystal size and size distribution control. U.S. Pat. No. 7,314,516 describes one apparatus and method that illustrates such a continuous crystallization process and the disclosure of this patent is hereby incorporated by reference.
One standard crystallization procedure involves contacting a supersaturated solution of a compound to be crystallized with an appropriate “anti-solvent” in a stirred vessel. Within the stirred vessel, the anti-solvent initiates primary nucleation which leads to crystal formation, sometimes with the help of seeding, and crystal digestion during an aging step. Mixing within the vessel can be achieved with a variety of agitators (e.g., Rushton or Pitched blade turbines, Intermig, etc.), and the process is done in a batchwise fashion.
Another known crystallization procedure for homogenous fluids employs temperature variation in a solution of a compound to be crystallized in order to generate a super saturation of the compound in the solution. Crystallization can then proceed from the super saturated solution.
It has been well known that ultrasound and cavitation can assist in nucleation of a compound within a fluid, although the mechanism is not completely agreed upon or understood. Subjecting a compound to be crystallized to cavitation energy is also known for generating nucleation sites and crystals of substantially uniform size. This is due, some think, to the high pressures and temperatures to which the compound is subjected in the immediate vicinity of the locations of cavitation bubble collapse.
A need exist for a continuous nucleation crystallization device and method that is capable of high throughput rates for extended periods of time yet that can produce nucleation and crystallization that is controllable and predictable. It is to the provision of such a device and method that the present invention is primarily directed.